This is a renewal R01 application to characterize an array of novel knockin (KI) mouse lines, we have created using a previously-described gene-targeting approach, to express unmutated precursors of four well- characterized, representative HIV-1 broadly neutralizing antibody (bnAb) lineages. Developing a preventative vaccine remains a critical priority to end the HIV pandemic, and it is widely held that such a vaccine will need to induce bnAb responses. While the eventual elicitation of bnAb responses in some HIV-1+ patients shows such responses are possible, no vaccine yet can elicit them. In this regard, animal models that can systematically identify impediments in bnAb precursor activation and maturation and iteratively test novel strategies to overcome such hurdles, would be highly beneficial for developing a bnAb-based vaccine. Three key roadblocks that merit further testing in such models are: i) B-cell tolerance controls, which can delete, inactivate, or modify bnAb lineage-expressing B-cells, before and during immunization, ii) high somatic mutation levels that accumulate in bnAbs over prolonged periods of infection, amounts prohibitive for vaccines to elicit, and whose exact level required for function is not known, and iii) no or minimal bnAb precursor affinity for standard Env immunogens. The overall objective of this proposal is to use novel bnAb precursor-directed immunogens to define minimal requirements to activate, and then induce breadth in, nave precursor KI B-cells from CH103, CH31, CH01, and CH58 lineages (and determine if/what host controls limit both processes). The matured Abs of these lineages are all relatively less mutated, making them more attractive vaccine candidates. Furthermore, they target distinct Env regions, and express a different set of functional traits/neutralization profiles, thus embody the bnAb spectrum. We hypothesize that this unique set of immunogens and models will allow us to learn how to induce individual, immunization-directed bnAb pathways with moderate breadth, manageable mutation levels and no (or only cryptic) self-reactivity. To test this central hypothesis, we will use a multilayered approach where in Aim 1, we will first define to what extent nave KI B-cells expressing CH103, CH31, CH01, or CH58 lineage precursors are under host controls. Then, in Aim 2, we will learn how best to activate these lineages with precursor-targeting immunogens, so they can be recruited into maturation pathways. Finally, in Aim 3, using immunogens that can initiate maturation, we will define minimal evolutionary trajectories to desired bnAb responses, under conditions where affinity maturation will be enhanced via cross-breeding KI mice to those overexpressing polymerase-? (with elevated Ig somatic mutation rates) and/or E-bcl2 (having increased B-cell survival). Learning how to induce, and what limits bnAbs in these model settings will help define more tractable vaccine targets and strategies to elicit in clinical trials. More generally, the host-HIV interaction issues addressed here (poor precursor binding, self-reactivity/mimicry, and excess mutation) may be informative for viral infections like hepatitis C or influenza, also requiring bnAb responses and problematic as vaccine targets.